The viscosity of a fluid varies with respect to spatial size and time
scales. For example, the tectonic plates can be viewed as an extremely
viscous fluid (or semi-solid) on human timescales, but they are much more
fluid/much less viscous on a timescale of ten million years (I think of
them sorta like lava lamps at this scale). Likewise, water is pretty thin
for humans but is very thick and syrup-y for micro-organisms.

So, should we include a sumti slot for the spacetime scale being
considered? I know that it seems like it might just be another condition
(and maybe it is), but consider this: water is thin for us but is, under
the same external conditions (temperature, pressure, no net motion or
shearing forces, etc.), thick for an amoeba. Is scale really a condition
(if so, it is a condition on the observer and not on the fluid - in this
example, the fluid is always water and the external conditions are
constant), or is it another direct property of viscosity?

Regardless of the conclusion, we should add either a sumti slot or a note
which details our choices.